In recent years it has become increasingly necessary to design transmissions, especially manual automotive transmissions, able to transfer both increased drive and synchronization power while at the same time reducing both transmission and transmission parts' sizes (see "Improved materials for synchronization rings", K. H. Matucha et al., Automobiltechnische Zeitschrift 83,5 (1981) , p. 227-230). As a consequence, transmission synchronization rings were designed with increasingly intricate shapes and small friction surface areas. The other member of the friction pairing, typically a friction cone, developed in the same manner. The friction power achievable per unit of surface area was also increased by the use of improved materials and surface structures and by reducing the dimensional tolerances. Both these changes improved the contact relationship between the two opposing friction surfaces (e.g. DE 25 38 882, DE 30 33 139, DE-PS 27 44 994, DE-GM 73 42 680).
Besides the need to reduce transmission size, there is also a demand to make shifting more comfortable while shortening shifting times. This requires the transmission be able to transfer more frictional work per unit of clutch surface area. However, as the amount of frictional work transfer increases, more friction heat will be produced per unit of time and surface area, and this heat must be dissipated. Another complication is that as the friction pairing parts make friction contact more rapid, the transmission oil present between the opposing friction surfaces must be removed more quickly. The oil is usually evacuated through drainage grooves and/or threads present in the friction surface.
Brass synchronization rings or resin-impregnated paper liners currently used in synchronization rings in automobile shifting transmissions have now generally reached their performance limits, as well as steel synchronization rings having ground, flame or plasma spray molybdenum coatings or custom molybdenum coatings which are used in trucks.
Alternatively, automotive friction clutches use sintered friction linings that are distributed on a sheet which is attached to the friction surface of a synchronization ring, typically by welding. These embodiments have substantially improved the performance of automotive transmission friction clutches (DE-PS 34 17 813). However, in view of the demand to use increasingly intricate designs in the synchronization rings, attaching by welding prefabricated sheets having a coating of sprinkled and sintered powder material presents the drawback of reducing the ring thickness and consequently, also reducing the force on the basic ring needed to cause mechanical cracking. Practically, however, the ring's mechanical strength should be increased because increased power is to be transmitted. Moreover, such rings are relatively costly to manufacture.
A clutch having optimal performance and durability can be made by employing different materials for the individual elements of the friction pairing, each material having different frictional characteristics. Certain friction materials possess particularly favorable friction characteristics; namely, they can deliver a great deal of frictional work through frictional contact and they are highly wear-resistant. Various brass alloys, molybdenum coatings, and a multitude of sintered coatings of various material compositions have all proven especially effective when used as these specific frictional materials. The specific rate of coating wear of the friction surface, or its reciprocal value, (the wear resistance of the two frictional materials) is determined in order to measure a friction pairing's durability. Friction pairing durability is reduced when a very abrasive friction surface is matched with a relatively soft opposing surface. Total wear behavior is also compromised when there are two abrasive, equally hard friction surfaces wearing against each other. In view of those observations, friction pairings were designed wherein specific attention was paid only to one of the two friction surfaces (the synchronization ring) of the pairing. Different matching cones were manufactured, most from types of steel having high surface hardnesses. The friction surface of the synchronization ring was machined, or in some cases made by spray metal coating, to give it acceptable surface structure properties.
Other attempts to improve automotive shifting transmission clutches have involved making the friction surface of the synchronization ring from hardened steel and coating the friction surface of the matching cone with a layer of molybdenum. At the same time, however, it was considered necessary to provide the friction surface of the synchronization ring with drainage grooves and/or threads for carrying off the oil. Because of these grooves and/or threads no significant advantage was obtained as compared with the combination of a Mo-coated synchronization ring in conjunction with a smooth matching cone. In particular, the problem of reduced synchronization ring strength due to the use of drainage grooves persisted.
For reasons well known to those having ordinary skill in the art of automotive transmissions, friction clutches must be lubricated with oil during operation. Consequently, a number of measures have been proposed to maintain a lubricating film without creating hydrodynamic bearing pressure between the friction surfaces, while still assuring the best possible coupling is had between the two friction surfaces. Accordingly, patent DE 27 44 994 proposes impregnating a paper-based friction lining with a synthetic resin in such a manner that it acquires a rough, porous surface. The patent elsewhere states "it has proven especially advantageous that the friction lining be porous and elastic. Due to its porosity, the friction lining can, during the synchronization process, absorb the oil film on the friction surfaces which would otherwise be an impediment, and carry it off during the operating cycle. " As a supplementary measure this patent teaches "it is effective to add axial grooves to the friction lining which serve to drain off the oil, and if the grooves have sharp edges, also to strip off the oil." However, when the elastic, compressible paper lining of this design is used, the pores are squeezed shut during the friction process.
Other references teach that oil displacement can be facilitated by roughening the friction surface. DE 28 34 840 suggests that point-shaped depressions should be made in the friction surface by means of spark erosion, creating a surface roughness R.sub.Z of 25-50 .mu.m. This technique is used rather than that described previously, namely roughening the friction surface by spraying on metal coatings or by mechanically sandblasting, or by chasing a fine, circular thread which is capable of taking up oil when the two friction surfaces make contact, thereby conducting the oil toward the axial grooves. According to the patent specification cited, the "roughening of the cone surface serves the purpose of piercing the oil film and rapidly establishing frictional contact." If these surface roughening techniques are used, better friction coefficients can be had than when the surface is roughened with a sprayed-on molybdenum coating or when a fine thread is applied to the friction surface. This roughening technique has not found acceptance in actual practice, though. The long manufacturing cycle time alone make it impractical for mass-produced parts such as transmission clutches. Any improvements in frictional characteristics as compared to the processes now being used are insignificant.